Im sorry to say but ibooks is crap. Before I upgraded my 3G to iphone 4 I was really anxious to have a look a the ibooks app but didnt want to upgrade the OS coz Id heard it might brick the phone. Anyway when I finally got to see the store, what a let down. There are hardly any books in there and it's all terribly over-priced.
Not suprising to see that Amazon are the cheapest. After doing some research myself I discovered that they were and this was one of the reasons why I have ordered the new kindle.
In some cases they are several pounds cheaper than their nearest rival.
For example I bought a book yesterday for £8.99 because I have been looking forward to reading it for a long time and didn't want to wait another month for the kindle. £8.99 is the cheapest price in the UK and elsewhere. The same book is priced at £5.30 in the kindle store.